kvm-unit-tests: add gcc version dependencies

kvm-unit-tests: add gcc version dependencies

On x86 (both i386 and x86_64), the kvm-unit-tests code uses
__builtin_reachable, which appeared in gcc 4.5. This commit adds the
relevant dependencies to kvm-unit-tests:

 - on i386, we need gcc 4.5 as the target compiler

 - on x86-64, we need gcc 4.5 as the host compiler, because we use the
   host compiler to build kvm-unit-tests for this architecture

Fixes:

  http://autobuild.buildroot.net/results/21340a7e6494e23adb2fe8dc6253ab220c380fec/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>